Supported chromium oxide catalysts have been employed to prepare various poly(.alpha.-olefin)s having good character for many purposes. Supported chromium oxide catalysts are used to prepare, for example, molding grade poly(.alpha.-olefin)s by means of a slurry polymerization system in which the polymers are produced in the form of small particles suspended in a hydrocarbon diluent. Among the supports employed for the supported chromium oxide catalysts have been silica alone or in combination with titania.
In the past, boron compounds have been employed in combination with the silica-supported chromium catalysts, such as the boranes and alkylboranes or boron hydrides as described in U.S. Pat. No. 3,484,428 (Kallenbach, Dec. 16, 1969).
The use of borane compounds with silica-supported chromium oxide catalyst is known, such as described in U.S. Pat. No. 3,947,433 (Witt, Mar. 30, 1976), producing a high shear response polymer. However, some undesirable side effects have shown up. For example, the use of trialkylborane as an adjuvant with silica-supported chrome oxide catalysts in ethylene homopolymerizations has been found to promote the formation of small, but significant and uncontrollable, amounts of monomers such as 1-butene, 1-hexene, and 1-octene, which then copolymerize along with the ethylene, resulting in a lowering of the normal natural polymer density which would have been achieved as a strictly homopolymer, and making very difficult the control of uniform polymer density consistently in a process.
Other supports employed with the chromium catalysts include aluminum phosphate-containing bases, activated phosphate-containing supports, phosphated silica-containing bases, and aluminum orthophosphate/silica combinations, U.S. Pat. Nos. 4,364,839, 4,364,840, 4,364,841, 4,364,842, and U.S. 4,364,855 (McDaniel and Johnson, Dec. 21, 1982). These patents describe the use of boranes, and boron alkoxides such as diethylboron mono-ethoxide, as cocatalysts with the aluminum phosphate-supported chromium catalysts.
The dialkylboron monoalkoxides behave in about the same manner as do the boranes when used with aluminum phosphate-supported chromium oxide catalysts in polymerization systems, tending to increase the density of the resulting polymer, increase melt index, and broadening the molecular weight distribution on the low molecular weight side. The trialkylboranes with AlPO.sub.4 -supported chromium oxides also increase the molecular weight distribution toward the low molecular weight side. Broadening of the low molecular side of the molecular weight distribution is a result not always desirable when a broader molecular weight distribution ethylene or other .alpha.-olefin polymer is desired due to undesirable molding characteristics.
A catalyst system which overcomes the above difficulties and which works well in .alpha.-olefin polymerizations is highly desired.